474 Royal Society ;— Prof. Maskelyne on New 



which I have given the name of Langite, in honour of my friend Dr. 

 Viktor von Lang, now of Gratz, and lately my colleague in the British 

 Museum. It occurs in minute crystals, or as a crystalline crust on 

 the Killas, of a fine blue with a greenish hue in certain lights. 

 The crystals are prismatic. The forms observed are (1 0), (0 01), 

 (110), and (2 1) & (0 1 0), the normal inclinations giving the fol- 

 lowing angles, which are the averages of many measurements : — 



110 1 10=56 16 

 100 1 10 = 61 52 

 001 201 = 51 46 

 conducting to the parametral ratios 



a:6:c= 1:0-5347: 0-6346. 



The crystals are twinned after the manner of cerussite, the twin axis 

 being normal to the plane (1 1 0). 



110 (110) ]" 10=112 33 

 100 (110) 10 0=123 44 

 110 (110) 110= 67 26 

 Cleavages seem to exist parallel to 1 and 10 0. The planes 

 1 and 10 are very brilliant. The plane of the optic axes, as seen 

 through a section parallel to the plane 1, is parallel to 1 0. The 

 normal to 1 would seem to be the first mean line, and it is nega- 

 tive. The optical orientation of the mineral is therefore b, c, a. 

 The crystals are dichroic. 



1 . Seen along axis c, c, greenish blue. 



6, blue. 



2. Seen along axis a, c, darker greenish blue. 



a, lighter bluish green. 



The specific gravity of Langite is 3*48 to 3*50. Its hardness is 

 under 3. It will not abrade calcite. 



Before the blowpipe on charcoal it gives off water, and fumes 

 and becomes reduced to metallic copper. Insoluble in water, it is 

 readily dissolved by acids and ammonia. Heated, it passes through 

 (1) a bright green, and (2) various tints of olive-green, till (3) 

 it becomes black. Water is given off the whole time, and finally 

 it has a strongly acid reaction. 



The first stage corresponds to the loss of one equivalent of water ; 

 the second reduces its composition to that of Brochantite ; at the 

 third it loses all its water. 



The chemical composition of Langite is represented by the for- 

 mula 3Cu" H' 2 2 + Cu" S0 4 + 2H' 2 O, which requires the following 

 numbers : — 



Calculated Average 



percentage. found. 



4 equivalents of copper 126*72 = 52*00 52*55 



4 equivalents of oxygen 32- =13*13 13*27 



I equivalent of sulphuric anhydride 40* =16*41 16*42 



5 equivalents of water 45- =18*46 18-317 



24372 100W \WW 





